Most geologists would answer, ‘The continents after the start of the Silurian Period’, and from now on they could be wrong. Evidence for an earlier ‘greening’ of the land comes from a detailed analysis of thousands of oxygen- and carbon-isotope measurements in Neoproterozoic carbonate rocks (Knauth, L.P. & Kennedy, M.J. 2009. The Neoproterozoic greening of the Earth. Nature, v. 460, p. 728-732). An important consideration in understanding the geochemistry of limestones is that however they originally formed as wet sediments at some later stage their constituents were largely transformed into crystalline aggregates by lithification through the intermediary of pore fluids. During lithification chemistry is equilibrated between crystals and the pore fluids, so if pore fluids are chemically (in this case isotopically) different from the sediment the resulting rock will have been changed isotopically. Studies of Cenozoic carbonates strongly suggest that the place where carbonate sediments are lithified most quickly is in coastal areas where terrestrial groundwater mixes with marine formation water in sediments. Since colonisation of the land by photosynthesising organisms groundwater C- and O-isotopes evolves in equilibrium with those organisms. The terrestrial biomass fixes 12C preferentially thereby depleting their proportion of 13C by up to 20‰. Groundwater, having originated as water vapour evaporated from the oceans that acts preferentially on 12O is also depleted in 18O. Consequently, low δ13C and δ18O signatures are passed on to groundwater and thence to carbonate rocks when groundwater participates in lithification.
Neoproterozoic carbonates plot in the same δ13C vs δ18O fields as those from the Phanerozoic. Earlier Precambrian carbonate data plotted in the same way show depletion in δ18O but not in δ13C, which signifies no terrestrial life, but normal preferential evaporation of 16O from the ocean surface to form rain and then groundwater. Knauth and Kennedy’s results suggest a strong likelihood that carbonates of the late Precambrian were lithified by groundwater from a land surface where photosynthetic organisms were well-established and abundant. There is likely to be a sceptical backlash to this remarkable conclusion, largely because it seems that the terrestrial biomass in the Neoproterozoic would have needed to be of the same order as that in later times. Yet molecular evidence from modern fungi, lichens, liverworts and mosses suggests that they evolved in the Neoproterozoic and Chinese scientists have found traces of what look remarkably like lichens in the 600 Ma Doushantuo lagerstätte – fungus-like hyphae and cells that resemble those of cyanobacteria (see The earliest lichens in May 2005 issue of EPN). In an earlier paper, Martin Kennedy had noted that around 700 Ma, the record of marine limestones show increasing 87Sr/86Sr ratios, suggesting an increase in the chemical weathering of ancient continental rocks. That may have coincided with biological agencies helping break down bare rock chemically to swelling clays that show a surge in Neoproterozoic sedimentary sequences (see Clays and the rise of an oxygenated atmosphere in March 2006 issue of EPN). The same paper pointed out that such clays increase the chances of preservation of buried organic matter, thereby boosting build-up of atmospheric and dissolved oxygen, as would terrestrial photosynthesisers. The feedback of increased oxygen to other eukaryotes that had evolved as heterotrophic animals would have enabled them to increase in size. Interestingly the earliest fossil animals occur in the same Chinese lagerstätte as the putative terrestrial photosynthesisers.
See also: Arthur, M.A. 2009. Carbonate rocks deconstructed. Nature, v. 460, p.698-699; Hand, E. 2009. When Earth greened over. Nature, v. 460, p.161.